The present invention relates to a printing quality examining apparatus (an apparatus for examining printing quality) used for decision as to whether printing quality is good or bad.
In a conventional printing quality examination method, as shown in FIG. 10, image data of printing paper are successively read in order of k=1, . . . , k=n-1, k=n by means of a camera and the image data are averaged in time to obtain estimated value data. In FIG. 10, i, j and k are marks for defining coordinates for picture or pattern data on paper. The estimated value data is compared with a previously read reference data and when a difference therebetween exceeds a threshold value, pixels thereof are decided to be defective. A flow chart of this decision operation is shown in FIG. 11.
In the conventional printing quality examination method shown in FIGS. 10 and 11, however, an estimated value data which is an average value of image data on current printing paper and image data on past printing paper is compared with the reference data in order to improve detection accuracy. Accordingly, once any defect occurs, estimated value data of normal pixels on several printing papers used subsequently to the occurrence of the defect are influenced by defective data in the past, so that the normal pixels are decided as defective pixels in error and consequently even satisfactory paper is discharged as defective paper.
Further, FIG. 12 illustrates a printing quality examination apparatus used heretofore. This conventional printing quality examination apparatus is now described with reference to FIG. 12. In FIG. 12, numeral 105 denotes a printing paper put on an impression cylinder A so that the printing paper is curved arcuatedly, numeral 114 a camera disposed in a detection unit, numeral 115 a xenon lamp disposed far from the detection unit, numeral 116 an optical fiber extended from the xenon lamp 115, numeral 117 a light irradiating end formed at an end of the optical fiber 116, and numeral 118 an optical axis of the light source. The camera 114 is disposed so that an optical axis of the camera is substantially vertical to the printing paper 105 and the light irradiating end 117 is disposed so that an optical axis of the light irradiating end 117 is oblique to the printing paper 105.
The printing paper 105 is irradiated by light from the light irradiating end 117, while image data on the printing paper 105 is taken in by the camera 114 and defective paper is detected from the image data on the printing paper 105.
However, in the illumination apparatus of the conventional printing quality examination apparatus shown in FIG. 12, the printing paper 5 is illuminated by illumination light from the single light irradiating end 117 and accordingly the xenon lamp 115 having the high luminous intensity is required. Furthermore, since the xenon lamp 115 is disposed far from the detection unit and light from the xenon lamp is led to the detection unit through the optical fiber 116, a manufacturing cost thereof is increased.
Further, a condensing lens or the like is used to feed the illumination light from the xenon lamp 115 to the optical fiber 116 effectively. The condensing lens or the like must be maintained for the optical deterioration and a great deal of labor is required therefor.
In addition, at a held end of the printing paper 105 on the impression cylinder A, when image data is taken in by the detection unit, an amount of light incident on the detection unit is varied due to fluttering of the paper, so that amendment of the illumination light amount is influenced greatly. More concrete description is given as follows.
When normal printing paper is irradiated by illumination light vertically and an illuminometer 119 is moved in parallel to the printing paper to measure the luminous intensity on the paper while the illuminometer is maintained vertically and in an equal distance to the printing paper as shown in FIG. 13, the luminous intensity on the paper is expressed by a curve named a substantially normal distribution and the distribution of the luminous intensity is maximized in the vicinity of the optical axis 120 of the illumination.
In the illumination apparatus of the conventional printing quality examination apparatus, as shown in FIG. 14, illumination light is incident along an incident axis 121 oblique to the vertical axis and passing through an incident point of illumination light and the light is reflected along an emitting axis 122 having the same angle in the opposite direction. Accordingly, the maximum point of the luminous intensity on the paper is offset on the side of the reflected light with respect to the vertical axis as shown by a curve of FIG. 14.
FIG. 15 illustrates variation in the luminous intensity on the printing paper 105 due to a fluttering of paper occurring at the held end of the printing paper 105 on the impression cylinder A.
When fluttering of paper does not occur at the held end of the printing paper 105, the distribution of the luminous intensity shown by solid line is obtained and the luminous intensity on the paper on the optical axis of the camera 114 is I.sub.0. The case where the held end of the paper is moved up is now considered. An amount of variation or movement is regarded to be able to be neglected as compared with a distance from the light irradiating end 117 and the camera 114 to the printing paper 105 and the printing paper 105 is assumed to be angularly moved or rotated about an intersection point O of the printing paper and the vertical line drawn from the camera 114 to the printing paper. The upward movement of the held end of the paper corresponds to the angular movement of the printing paper 105 in the counterclockwise direction by an angle .alpha..
At this time, the normal line extending from the intersecting point 0 vertically to the printing paper 105 is rotated by .alpha. in the counterclockwise direction similarly. Further, the emitting axis of the reflected light is rotated by 2.alpha. and the maximum point of the luminous intensity in the distribution of the luminous intensity on the printing paper 105 is also moved leftward as shown by broken line in FIG. 15. Thus, an amount of light received by the camera 114 is varied from I.sub.0 to I.sub.1. This corresponds to a variation in the amount of light received by the camera 114 due to fluttering of paper.
Further, FIG. 16 schematically illustrates a circuit configuration of a conventional printing quality examination apparatus. In this case, an image of a printing paper having the luminous intensity on the surface thereof maintained constant by illumination light from an illuminating light source is taken in by a line camera of a detection unit 201. The luminous intensity on the printing paper is maintained to be constant and decision as to whether the printing paper is good or bad is made as follows.
As shown in FIG. 17, an output signal from the detection unit 201 is reduced in substantially inverse proportion to a machine speed. The output signal from the detection unit 201 is supplied to an amplifier 202 of FIG. 16 to be amplified and the amplified signal is supplied to an analog-to-digital (A/D) converter 203 to be converted to a digital signal. Thereafter, the digital signal is subjected to a correction process with respect to the machine speed and is held to be a fixed signal level as shown in FIG. 17. Then, the digital signal is supplied to a comparison operation circuit 204 of FIG. 16 to be compared with a previously taken-in reference image data. This operation result is supplied to a decision circuit 205 in which decision as to whether the printing paper is good or bad is made.
The decision result is supplied to the control and display unit 206 in which unsatisfactory paper is discharged and an alarm to an operator is displayed.
However, the printing quality examination apparatus shown in FIGS. 16 and 17 has the following problems.
The luminous intensity on the printing paper is maintained to be constant by illumination light from the illumination light source and since an amount of light received by the printing paper is reduced when the machine speed is increased, a gain is applied to the whole signal in order to maintain the received light amount constant. Accordingly, there are the following problems.
(1) Since a gain is applied to the whole signal, even noise is amplified. PA1 (2) Variation of the received light amount depending on quality of paper cannot be understood. PA1 (3) Since an amount of light is reduced due to a life of the illumination light source, it is necessary to maintain the illumination light source.
Further, in the conventional printing quality examination apparatus, as shown in FIG. 18, an image data of a blank portion 311 of the printing paper is taken in by the camera of the detection unit and brightness of this portion is compared with a previously set reference value of the luminous intensity. An amount of light of the illumination light source is corrected on the basis of the result thereof to thereby obtain the luminous intensity required in the detection unit.
However, in the conventional printing quality examination apparatus shown in FIG. 18, it is presupposed that the blank portion 311 for correction of the light amount is present in the printing paper. However, some printing papers have no blank portion and in this case, it is impossible to correct the light amount.
On the other hand, when the blank portion 311 necessary for the correction of the light amount exists in the vicinity of a picture portion 312 and the picture portion is moved or shifted due to mechanical vibration or the like, an amount of light in the printing portion other than the blank portion 311 is sometimes detected. When the light amount is corrected in such a state, there is a problem that a corrected light amount in the detection portion becomes instable.